Polystyrene foam is a material which is widely used as a highly protective and light-weight shipping container and as a low-cost material providing good thermal insulation in various types of panels used in the construction and appliance industries.
Foamed polystyrene is produced using styrene polymer crystals which are run through an extrusion apparatus. In general, the extrusion apparatus consists of a melt section to which the crystals are introduced and raised to a melt temperature of about 425.degree. F. and a number of pumping sections or stages to which the melt is passed. Some of the pumping sections are equipped to introduce additive materials to the melt during its passage therethrough. During passage through the final pumping sections prior to passage through the forming or shaping head a blowing agent, such as carbon dioxide, is introduced in sufficient quantity to impart the desired density to the final product. During passage through the various pumping sections the material is cooled to the optimum extrusion temperature--usually about 350.degree. F.
Foamed polystyrene burns readily. This property is not considered a disadvantage where it is used as a packaging material. However, where it is used as thermal insulation in a construction panel or in an appliance such as a refrigerator its combustion properties are very objectionable. Conventional flame-retardant additives which have been used in the past have contained halogenated hydrocarbons, antimony oxide, zinc borate and talc. The present invention is directed to the production of a new and improved polystyrene foam which is flame-retardant.